How to allow member to write posts
Ultimate Member displays posts and comments on Profile page, but it doesn't manage permissions and capabilities to write posts and comments for default roles. Member must have role Author (author) or have the same capabilities as Author to write posts. Ultimate Member can edit capabilities for roles, that belongs to Ultimate Member. Use third-party plugins (such as User Role Editor) to edit capabilities for roles, that doesn't belong to Ultimate Member.
Role, that has these capabilities, can write posts. You also should allow this role to use admin panel. Member may create post using default WordPress tool [wp-admin > Posts > Add New]
[Ultimate Member > User Roles > Edit Role]
[wp-admin > Posts > Add New]